The present invention relates to a transmitter and, more particularly, to a transmitter of the type using a dual conversion system which subjects a modulated signal to frequency conversion twice to produce a high frequency signal to be transmitted.
A transmitter using a dual conversion system is disclosed in, for example, Japanese Patent Publication No. 43609/1987 (JP-B2-62-43609). The transmitter disclosed in this Patent Publication has a single local oscillator. Specifically, one of two frequency converters is supplied with a local oscillation signal from the single local oscillator while the other is supplied with a multiplied local oscillation signal. Such a single local oscillator scheme is successful in reducing the circuit scale of a transmitter.
In a very small aperture terminal (VSAT) system, a frequency band to be used is allocated beforehand. The allocated frequency band is subdivided into a plurality of frequency bands, and each of such frequency bands is assigned to a respective one of VSAT station. A hub station sometimes control the frequency band assigned to each VSAT station, depending on the traffic. Therefore, each VSAT station has a variable frequency local oscillator so that the transmission signal frequency may be varied beforehand.
The conventional transmitter using a dual conversion system and having a single local oscillator as stated earlier is applicable to the VSAT system. However, at a VSAT station, a modulated signal from an indoor unit (IDU) which includes a modulating section is inputted to an outdoor unit (ODU) by a cable. Since the cable loss increases with the increase in the frequency of the modulated signal which is propagated through the cable, the frequency of the modulated signal should preferably be low. In this condition, the leakage component of the local oscillation signal has a frequency close to the frequency of a frequency-converted intermediate frequency (IF) signal, as will be described later in detail. In addition, the frequency of the frequency-converted IF signal is close to the frequencies of spurious components. Further, use is made of a band pass filter having a broad pass band, so that a plurality of frequency bands may be used. It follows that the band pass filter connected to the output of the frequency converter and having a fixed band width has to have a sharp cut-off characteristic at the outside of the band. However, a band pass filter with a sharp cut-off characteristic is expensive and, moreover, difficult to produce.